Normally in vehicles using a fixed capacity compressor in the air conditioning refrigeration section of a heater-ventilation, air-condition (HVAC) unit, an evaporator operates with respect to a fixed steady-state, evaporator surface temperature set point. This set point establishes a temperature reference about which the evaporator provides the coldest downstream air without the downstream surface fins of the evaporator developing frost due to the humid inlet air being warmer than the evaporator fin surface causing water to condense from the air and freeze.
An evaporator temperature probe mounts at a chosen spot to measure the downstream side of the evaporator to monitor changes of evaporator temperature with respect to the set-point temperature. The probe detects thermal changes on the downstream surface of the fins respect to the steady-state set point. Electrical resistance of the probe decreases as downstream surface temperature of the evaporator increases. Hence, an analog logical signal at an output terminal is used to indicate when the evaporator temperature rises above or falls below the set point by, illustratively, 1.degree. F. This evaporator set point signal usually set in the factory to within a range suitable for normal weather conditions in most regions of the country, provides a signal to a control circuit for turning ON the fixed capacity compressor when the evaporator temperature exceeds the set point and turning OFF the compressor when the evaporator temperature goes below the set point. This cycling ON and OFF the compressor helps prevent the build-up of frost upon the downstream surface of the evaporator fins. However, due to the wide ranges of temperature and humidity in the various regions of the country, such a system usually does not operate efficiently in regions that experience cold or extreme hot temperatures.
In an effort to optimize the operation of the refrigeration section of the HVAC unit so that the evaporator operates without developing frost for all weather conditions, a search was initiated to find other schemes for controlling fixed capacity compressors. That search ended in the improved refrigeration section presented in the instant invention.